Every endeavor has been made for higher sensitization and reduction of residual colors after processing of a silver halide photographic material. It is known that a sensitizing dye which is used for spectral sensitization largely affects capabilities of a silver halide photographic material. A trace of structural difference of a sensitizing dye largely affects photographic capabilities such as sensitivity, fog and storage stability but it is difficult to foresee its effect. Many engineers have hitherto synthesized various kinds of sensitizing dyes and endeavored to investigate photographic capabilities thereof but it is not possible to know photographic capabilities in advance yet.
Further, a trial of reduction sensitization for higher sensitization of a silver halide photographic material has been studied heretofore. Various compounds are disclosed in many patent as a useful reduction sensitizer, for example, tin compounds are disclosed in U.S. Pat. No. 2,487,850, polyamine compounds in U.S. Pat. No. 2,512,925, and thiourea dioxide based compounds in British Patent 789,823. Further, in Photographic Science and Engineering, Vol. 23, p. 113 (1979), comparisons of natures of silver nuclei formed according to various reduction sensitizing methods are described and dimethylamineborane, stannous chloride, hydrazine, and methods of high pH ripening and low pAg ripening have been employed. Reduction sensitizing methods are also disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777 and 3,930,867. Not only the selection of reduction sensitizers but contrivances of reduction sensitizing methods are disclosed in JP-B-57-33572 and JP-B-58-1410 (the term "JP-B" as used herein means an "examined Japanese patent publication").
However, it has been found from the studies of the present inventors that when reduction sensitized silver halide grains are spectrally sensitized by the adsorption of sensitizing dyes, in particular, when spectral sensitization is conducted in green and red regions, it has been extremely difficult to obtain sufficient spectral sensitivity without being accompanied by disadvantageous reactions on photographic capabilities (e.g., the increase of fog).
Moreover, a method of adsorbing a sensitizing dye at high temperature (50.degree. C. or more) for preventing the desorption of a sensitizing dye from silver halide grains in a photographic material (in particular, under high humidity conditions), and a method of adsorbing a sensitizing dye before chemical sensitization for high sensitization are widely known, but if these methods are applied for adsorbing a spectral sensitizing dye in green and red regions onto a reduction sensitized emulsion, fog is markedly increased.
From the above-described reasons, techniques of spectrally sensitizing a reduction sensitized silver halide grains in a high sensitive state and without causing malefic effects such as the increase of fog have been demanded.